Actually, it's perfectly safe for people to marry their first cousins--there's only a 2% increased chance of risk of problems in children of two first cousins than of unrelated people--that's not much! Odds are the children of first cousins are going to be healthy--any laws against first cousin marriage predate modern genetics, most places actually allow first cousins to marry anyway, with approximately 20% of couples worldwide being first cousin-couples--and there were many marriages b/t first cousins in the past--odds are YOU are descended from a first cousin pairing! The social stigma against them is really unfounded, there's nothing wrong with it! Read up on the facts of first cousin marriage here:

http://www.cousincouples.com/?page=facts

Sorry, it's just that I'm a big A/R shipper and so I've been reading up on it--and considering how inbred the wizarding world is anyway, with those purebloods only marrying purebloods (like even the Weasleys), A/R's children would really be better off than say Scorpius and the like anyway genetically speaking!